Assignment, by proton magnetic resonance, of histidines of dihydrofolate reductase from chicken liver

The effects of pH and temperature upon C epsilon 1 H resonances of the four histidyl residues of chicken liver dihydrofolate reductase in binary complex with methotrexate were studied by 500-MHz 1H NMR spectroscopy. The four histidines labelled a, b, c, d are distinguishable by their pK values and the chemical shifts of their C epsilon 1H protons. The local electromagnetic environment as deduced from X-ray studies at 2.9 A resolution was used as a basis for proposed assignment of the four histidines. The assignments were a: H42, b: H140, c: H131, d: H87. Furthermore the histidyl residue labelled c was shown to be upfield shifted in its C epsilon 1H proton in the enzyme-methotrexate complex compared to the native enzyme. The hypothesis of a conformational change of the protein is discussed.